The present invention relates to fiber optic gyroscopes (hereinafter referred to as “FOG”), and more particularly, to structures that support and stabilize the coils of optical fiber within a FOG.
A FOG is used to measure the rate of rotation of a vehicle or other host platform to which the FOG is attached. The FOG typically includes a coil of optical fiber that is wound about a bobbin. The coil, along with the bobbin foundation, can rotate about an axis of rotation. A light source transmits light into each end of the optical fiber, so that two light transmissions propagate through the optical fiber in counter rotating directions. Detection circuitry, typically residing within an Integrated Optical Circuit (hereinafter “IOC”), receives the light transmissions as they emerge from the ends of the optical fiber and measures the relative phase relationship of the light. The phase relationship of the two light transmissions is related to the angular rotation of the FOG coil about the axis of rotation, and may be used to derive an output that is indicative of the rate of rotation of the FOG coil.
FIG. 1A shows a perspective view of a typical prior art bobbin, and FIG. 1B shows the prior art bobbin of FIG. 1A in cross-section view. The bobbin 10 is cylindrically shaped and has an upper flange 12 and a lower flange 14 disposed on opposite ends of the bobbin. The bobbin 10 further includes a principal axis AX, that is perpendicular to the planes formed by the outer surface of the upper flange 12 and the outer surface of the lower flange 14. The principal axis AX is an axis of rotation about which the bobbin and optical coil assembly rotate. The upper and lower flanges are typically characterized by constant thickness, and fabricated as thin as practical, to maximize the volume available for optical fiber.
To stabilize the optical fiber on the bobbin so that the FOG can operate in high vibration environments, the optical fiber is often wound onto the hub with an epoxy adhesive between the hub and the first layer, and also between subsequent layers of optical fiber. Once the optical fiber is completely wound onto the bobbin, the coil assembly is placed in an elevated temperature to cure the epoxy. Because the optical fiber restrains expansion of the coil in the radial direction, the thermal expansion of the coil is greater in the axial direction than in the radial direction. Large thermal induced stresses are therefore produced in the bobbin material and fiber pack when the bobbin is exposed to a temperature different from the epoxy curing temperature, which is the minimum stress temperature of the coil. The effect of changing the temperature of the coil is best illustrated by a finite element model in FIG. 2, which shows the predicted stresses generated in a bobbin manufactured to a prior art. In this example, the zero stress temperature is +85 degrees Celsius and the stresses are calculated at the lowest operating temperature of the coil −54 degrees Celsius. Due to the relatively large thermal expansion of the epoxy, at temperatures below the zero stress temperature, the bobbin flanges are placed in bending, generating large stresses in the flange at the interface with the hub.
A disadvantage with the prior art bobbin configuration shown in FIG. 1A is that the flanges 12, 14, extend from the bobbin 10 without supplemental structural support. Further, since the prior art flanges 10, 12 are typically relatively thin to maximize the volume available for the optical fiber, these flanges have deformed and cracked as a result of thermal induced stresses. Further, the thin prior art flanges are typically characterized by natural modes of vibration at relatively low frequencies. These low frequency modes of vibration result in susceptibility to the shock and vibration environments the host platform experiences.
To meet performance requirements over temperature, the optical fiber coil used in fiber optic gyroscopes is often wound in a quadrupolar pattern. For optimum thermal compensation, the total number of layers is a multiple of four. Although other winding patterns may be used, the winding process typically involves the winding of a predetermined length of optical fiber equally onto a first feed spool and a second feed spool, so that the midpoint of the optical fiber occurs between the two feed spools. Winding commences at the midpoint of the fiber and the first layer is wound using the first feed spool. For the quadrupolar pattern, the second layer and the third layer are wound using the second feed spool, and the fourth layer is wound from the first feed spool. This four-layer pattern is repeated until the requisite number of layers has been wound onto the coil. Adaptations of this winding process involve methods in which the fiber transition occurs between two non-adjacent layers, for example between the first layer and the fourth layer. Another disadvantage with the prior art bobbin configuration stems from the fact that the flanges 12, 14 extend perpendicularly from the bobbin 10. Some automated coil winding systems require the bobbin to have a slot in each flange that extends in a radial direction from the principal axis to the outer edge of the flange. These slots are both costly to machine and significantly reduce the stiffness and hence stability of the bobbin. A guide wheel carries the fiber from the feed spool to the bobbin. To ensure precise placement of the fiber and to minimize fiber crossovers, the guide wheel must be situated close to the fiber layer being wound. With bobbins of the previous art (having parallel flanges as shown in FIG. 1A) the fiber from the idle, non-winding feed spool must temporarily exit the slot to prevent interference with the guide wheel. The fiber from the idle feed spool is thus “parked” in a position outside the flange until that feed spool is required for winding, at which time the feed spools change places, and the previously-active feed spool is parked while the previously-idle spool winds its fiber onto the bobbin. This process continues, with the feed spools alternating, until the fiber coil is completed.